487 



lands above. The elevated deposits of Spoon River and the 

 consequent crowding of the channel of the river to the east 

 bluff at Havana force all of the water of overflow (at stages 

 below about 16 feet) to seek the main channel. The configura- 

 tion of the low-lying bottoms above is such (see PI. II.) that 

 the lakes in question form natural channels for the movement 

 of a large body of impounded water. This movement is well 

 marked at stages above eight feet. So far as I am able to judge 

 from field observations, the current conditions in Thompson's 

 Lake and the Dogfish-Quiver area are not greatly different. 

 The current continues in both lakes as levels fall to six feet, at 

 which level Thompson's Lake loses its connection with the 

 river through the " cut road " (PI. II.), and movements in it 

 at lower levels are confined to those due to ingress and egress 

 of water through the slough, and are consequently inconsider- 

 able. On the other hand, Quiver Lake continues to be traversed 

 by the discharge from Quiver Creek, and our collections were 

 usually made in the channel in the vegetation. In Dogfish 

 Lake at low stages there *is no current traversing the lake. 

 Phelps Lake lies at so high a level that only the floods exceed- 

 ing eleven feet bring it into connection with the general cur- 

 rent of overflow, which in this case generally comes from 

 Spoon River. Below this level the only movement in its water 

 is the gentle one due to the receding flood. So far, then, as 

 the current is concerned, it is a common though not equally 

 distributed factor at high- water stages in all areas compared, 

 while at low water it is an important feature in the environ- 

 ment in Quiver Lake but is practically absent in the other 

 three localities. This fact undoubtedly accounts in part for the 

 barrenness of the waters of Quiver Lake (1.53 cm. 3 per m. 3 , or 

 only .55 for the average of low-water periods — i. e. below 5 ft.) 

 as compared with those of Dogfish (4.22), Thompson's (8.13) and 

 Phelps (19.44) lakes. This current does not, however, traverse 

 or appreciably affect the waters of Dogfish Lake, and their bar- 

 renness still remains for contrast with the productiveness of the 

 vegetation-poor waters of Thompson's and Phelps lakes. 



